Odd deceleration rattle

[Deschodt]

Been searching and so far nothing... My 2003 SMG started making a 2 second rattle whenever I come to stop in gear. Most noticeable in 3rd -> stop, the second the revs pass around 1800 rpm (towards idle), rattle... Sound like a pre-"golf tee" fix E46 but the M3 doesn;t seem to have those valves.

I put it on my ramps and banged on all the exhaust parts (all the while hoping for no earthquake), shields, everything seems tight...
The engine is smooth and rattle free in neutral, up and down... only happens coming to a stop in gear as revs drop... 1800 rpm is my guess..

Any other gotcha ?

[dworthy]

Have you looked at the bolt for the timing chain guide that sits behind the thermostat housing?

[Deschodt]

Mechanic found it. It only happened when warm and at the range I described. Initially we feared a bad catalytic converter (back ordered and $$$$) but it turned out to be the joining of 2 exhaust pipes - the metal “gasket” is inside ans tightened from outside with a clamp and it was rattling only when the metal pipe expanded and there was more room for it to move. It’s gone now - so much nicer !